The present invention relates to a drive unit system including a flywheel which assists braking by storing energy during braking and from which energy is recovered at all other times. The drive unit system includes a drive engine, a gear shift transmission, and a flywheel accumulator. The flywheel accumulator helps accelerate a vehicle, as it is discharged, i.e. giving off rotational energy. Furthermore, the flywheel accumulator is used to brake the vehicle, whereby it is charged, i.e. takes up rotational energy. The braking energy is thus usefully recovered. Such drive units are intended, in particular, for city buses but are also suitable for short-haul rail vehicles.